They’re not a presidential election. Trump won the US presidential election on November 8, 2016 and isn’t up for re-election until 2020.WHAT’S THE STATE OF PLAY?

Congress is made up of the House of Representatives (lower house) and Senate (upper house) – and the Republicans currently control both.

Up for grabs this year are all 435 seats in the House of Representatives. Elected representatives serve two-year terms.

To control the House, a party needs 218 seats. The Republican party holds a comfortable majority of 235 seats.

The Democrats need to flip 24 Republican seats while keeping the 194 seats they currently hold. According to American politics site FiveThirtyEight, Democrats have around an 80 per cent chance of success.

A voter marks his ballot at an early voting polling station at West Los Angeles College in Culver City, California. (AAP)

There are 35 seats up for grabs, with analysts predicting the Republicans will hold on to 50 seats and retain their majority.

But the Democrats need to hold on to their seats and gain at least two more to clinch the majority.

WHY DO MIDTERMS MATTER?

First up, whichever party controls Congress controls the passage of legislation.

But the 2018 midterms also matter because many see them as test of Trump’s presidency. If Republicans retain the majority, Trump will be able to push through his desired tax cuts, anti-abortion policies and controversial immigration program.

But if the Democrats win back control of the House of Representatives, they could not only thwart Trump’s legislative agenda but use their numbers to set up select committees to investigate him.

- FLORIDA (House): National Republicans and Democrats are pouring major resources into the Miami-area 27th District seat, held since 1989 by retiring Republican Rep. Ileana Ros-Lehtinen. The Democratic nominee, Donna Shalala, has been backed by Hillary Clinton. But she's facing a stiff challenge from her Republican opponent, Maria Elvira Salazar, a Cuban-American. Trump won Florida in 2016, but Clinton won this congressional district by nearly 20 points.

- KENTUCKY (House): The ruby-red state known for the Derby and sweet bourbon is hosting one of the most competitive and expensive races in the country. The Lexington-area battle pits third-term Republican Rep. Andy Barr against Democrat Amy McGrath, a retired Marine fighter pilot. Trump won the 6th District by more than 15 percentage points in 2016.

- TEXAS (Senate): Republican Sen. Ted Cruz is seeking a second term against Democratic Rep. Beto O'Rourke, a rising star in the Democratic Party who has shattered Senate campaign fundraising records despite shunning donations from outside political groups. O'Rourke is trying to become Texas' first Democrat to win state-wide office since 1994, while Cruz has made nice with Trump.

- ARIZONA (Senate): Democrats have high hopes for flipping this seat in Arizona, where Democratic Rep. Kyrsten Sinema is running against Republican Rep. Martha McSally. They are running for the seat left open when Sen. Jeff Flake, a sharp critic of Trump, opted to retire, acknowledging that he could not win a Republican primary in the current political climate.

- IOWA (House): One Iowa race offers a test of whether a Trump-style advocate for immigration limits can win. Republican Rep. Steve King is keeping a low profile in his bid for a ninth House term, his success suddenly in question after he was engulfed in controversy for his support of white nationalists. But Democrats, already hoping to flip two other seats among Iowa's four-person delegation, have a tough road to success in the 4th District that voted for Trump by 27 percentage points.

WHEN WILL THE RESULTS BE ANNOUNCED?

Counting in each state begins as soon as polls close, which will be between 6pm and 11pm EST on November 6 across the US.

In Australia, this means between 10am to 3pm on Wednesday, November 7.

We can probably expect to see the first states declaring results by midnight local time.

WHAT WILL THE RESULT MEAN FOR AUSTRALIA?

Australia was on the back foot when Trump defeated Clinton, his Democratic rival, in the 2016 presidential election.

There was that ”unpleasant” phone call between Trump and then Australian prime minister Malcolm Turnbull over the asylum seeker deal. And Turnbull didn’t even have Trump’s mobile number, forced to ask Trump’s golfing pal Greg Norman for it.

But in spite of the rocky start, Australia has emerged during the Trump presidency better than perhaps any other ally.

In June, Trump announced only Australia and Argentina would not be hit by his 25 per cent steel and 10 per cent aluminium tariffs.

If Democrats do take control of the House, Trump's stalled infrastructure plan - something Democrats liked - could re-emerge on his agenda.

Malcolm Turnbull and Donald Trump. (AAP)

Australian Ambassador to the US Joe Hockey and Australian companies, with their history of asset recycling, are jumping out of their skins to get involved.

If Trump does lose control of Congress, the biggest impact on Australia will likely be instability to world markets.

There are fears Trump, with his eyes solely set on winning the 2020 presidential election, will lash out and concentrate on his America First policy.

There is the real prospect of a government shut down over his demand for funding of a US-Mexico border wall.

Trump has already been criticised for skipping this month's trip to the East Asia and ASEAN summits in Singapore and the Asia Pacific Economic Cooperation meetings Papua New Guinea.

Democrats, with control of one chamber, and Republicans in the other, are expected to lead to legislation gridlock.